As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 5596 - STL - CREW REPORTED THAT HYDRAULIC SYSTEM NR 1 QUANTITY WAS DROPPING DURING FLIGHT AND WAS FOLLOWED BY A LOW QTY MESSAGE ON THE EICAS. MAINTENANCE REMOVED AND REPLACED AN O-RING ON THE PRESSURE TRANSMITTER. SYSTEM OPS CHECKED GOOD AFTERWARDS.